Using groups in other files
If you go to File > Append or press ShiftF1 , you can navigate to the blend file, open the Group folder and import the group.



Appending the group will create a unique copy of it in the new file. If you want to keep working on the group in the original file and have any changes there be updated in the new file, you should Link instead. Go to File > Link or use CtrlAltO. Note that if you link however, you won't be able to edit the linked group.

Related - How to add an object from another scene?.